Patent GB-521949-A (GB)
Title: Transmission of graphic or pictorial material over submarine cables
IPC: H04N1/00
Filed: 1938-11-28
Abstract: 521,949. Facsimile transmission systems; valve amplifying circuits. TRIGGS, W. W. (Western Union Telegraph Co.). Nov. 28, 1938, No. 34560. [Classes 40 (iii) and 40 (v)] In a method of transmitting picture telegraph signals over submarine cables, the modulated current representing the picture is converted into direct-current positive 'and negative signals varying in amplitude with gradations of picture density, the received currents being reconverted into an alternating current to reproduce the picture. In the system described, picture signals are transmitted over a system including balanced land lines, submarine cables, and repeaters as shown in Fig. 1. Transmitting - station. - At the sending station A, Fig. 2, light from a source 7 scans the picture 8 wrapped on a drum 3 which is rotatably and slidably mounted on a shaft 5 driven from a motor 4. An overlap portion of a given tone value, say grey, is provided along the length of the drum to transmit a signal corresponding to zero current once per revolution of the drum. This is utilized at the receiver to correct for zero wander in the cables. The reflected light passes through the chopper device 9 to a photoelectric cell 12. The picture modulated signals Fig. 7a, pass through the amplifier 2 to the rectifier 14 which is divided into two halves to minimise capacity unbalances. Networks 16, 17 suppress ripple in the rectifier output, Fig. 7b, and improve the shape of. the wave. The amplifiers 18 send the waveform, Fig. 7c, to line. Batteries 20, 20<SP>1</SP> compensate for the fact that the potential of point a is always above that of point b. Line-to-line repeater.-A repeater, Fig. 3, connected between two sets of metallic lines, comprises an amplifier system of the type described in Specification 149,858, [Class 40(iv).] High-frequency components are selected by the transformer 27 and its associated network 25 and amplified, the lower-frequency components being byepassed by the networks 31. Specification 153,357, [Class 40 (iii)], also is referred to. Line-to-cable repeater.-A repeater, Fig. 4, is connected between a metallic line and a grounded submarine cable. The signals are applied to the balanced gain control and shaping network 35, which includes direct coupling 38 for the , lower-frequency components and transformer coupling 36 for the higher-frequency components. The valve 43 amplifies loop currents only, the longitudinal currents being suppressed by the action of valve 44. The current through the common self-bias resistance 48 remains substantially constant when loop' signals are applied since valves 43 and 44 are fed in antiphase. When. longitudinal currents flow, the voltage across valve 43 decreases, say, and that across resistance 48 increases since the valves 43, 44 are then fed in phase and there is no change in the output voltage across the potentiometer 49. The battery 52 neutralizes the excessive self bias developed across resistance 48 when the switch arm 50 is on the contact 54. The output of valve 43 is fed to amplifiers 55 and transmitted through shaping network 58 to the cable 60. The blocking condenser may be replaced by a battery if it is desired to obtain a conductive connection to the cable. Cable-to-cable repeater.-A repeater, Figs. 5a, 5b, between two submarine cables uses balanced tetrode circuits with distortion compensating and interference suppression features. A feedback curcuit 64 from the third stage to the anodes of the second stage increases the amplification of the lower-frequency components. A large resistance 70 in the cathode circuit of the third amplifier stage has a degenerative effect on longitudinal currents and no effect on loop signal currents. The output circuit of this amplifier is returned to the negative terminal of the battery 72 so that a relatively small grid bias battery suffices for the succeeding stage. A gas discharge tube 75 protects the cable from excessive potential. Cable-to-line repeater ; receiving station.- Figs. 6a, 6b, show cable shaping network, three stages of amplification and the receiving system together with apparatus correcting for the zero wander of the cable. The amplified output is applied from the valves 78, 79 across the anodes and cathodes of the valves 90, the output from a 2500 cycle/sec oscillator being injected into the grid circuit. The output, Fig. 7f, is amplified and applied to the recording stylus 97. The receiving drum rotates synchronously with the transmitter and recording occurs as varying current passes through the sensitized paper 8. The circuit is arranged so that at least a small current always flows through the paper. The setting of the potentiometer 116 determines the amount of contrast and the position of the switch 98 whether the picture recorded is a positive or a negative. A 60-cycle/sec current is also injected into the stylus circuit as relay 89 operates to produce a set of dots on the overlapping portion of the paper once each revolution to provide a check on the synchronization of the system. , Correction for cable zero wander.-At the end of each revolution of the transmitting drum the output of the valves 78, 79, Fig. 6b, should be zero. The screen potential of valve 78 is controlled by valve 81, in the grid circuit of which are connected the left-hand contacts of relays 84, 85. A commutator 82 rotates in synchronism with the transmitter to vibrate the armatures of relays 84, 85 once per revolution. Their right-hand contacts are connected across the output of the receiving amplifier circuit so that the condenser 87 is charged in accordance with its output. This charge is transferred to the large condenser 86 when the relay moves over to its left-hand contacts and hence controls the potential of valve 78 to give zero output from the amplifier corresponding to the zero input to the system each revolution. If necessary correction can be applied to valve 79 also, Fig. 6b, (not shown).